08/21/2012 at 01:00 PM EDT
Justin Theroux has a lot to smile about
The newly engaged actor
joined three friends for dinner at SoHo eatery The Smile in New York City Monday night.
Although he was without his fiance, Jennifer Aniston
, Theroux, 41, was "beaming all night" an onlooker tells PEOPLE.
The four friends – who ordered up healthy fare like salads, chicken and trout – looked relaxed and carefree during their three-hour dinner. "Justin was laughing so loud the whole restaurant could hear him. [They were] obviously having a great time," the source says.
At one point in the evening, the pals raised their glasses – Theroux's was filled with water – and toasted to each other.
"Justin looked like the happiest guy in the world," the source adds.